(A) Field of the Invention
The present invention relates to a refractory insulation board wherein refractory fibers are held in the shape of a sheet or board by means of a binder composition.
(B) History of the Prior Art
Binder compositions in the prior art for binding refractory fibers were frequently not entirely satisfactory. Many such binders contained organic materials such as the binders disclosed in U.S. Pat. Nos. 3,835,054; 3,961,968 and 3,944,702. Such organic compounds are highly undesirable since they decompose at high temperature conditions frequently encountered by the insulation material.
Other totally inorganic binders, while able to withstand high temperature environments, were undesirable since during drying and curing of the fiber article, the binder tended to migrate thus causing excessive binder buildup in some areas of the article and causing the presence of insufficient binder in other areas of the article. A common example of such inorganic binders is a colloidal silica such as disclosed in U.S. Pat. Nos. 3,649,406 and 3,976,728.
It has been disclosed in U.S. Pat. No. 3,935,060 that a particular process for drying a mostly inorganic binder comprising colloidal silica and some hydrolyzed ethyl silicate can be used which avoids binder migration. The process comprises heating the article in a microwave oven to instantly evaporate the water before migration has a chance to occur. The process disclosed in U.S. Pat. No. 3,935,060 is not particularly desirable since complicated and costly microwave equipment is required. Furthermore, unless very powerful and potentially harmful microwaves are used, some migration is still believed to occur.
It has been suggested in U.S. Pat. No. 3,551,266 that migration of a silica sol (i.e., colloidal silica) binder could be prevented by the gelation of the silica sol. Such a system is useful in refractory cement containing refractory fibers, when the cement is to be rapidly applied to a particular substrate. Unfortunately, the system has been inapplicable to the production of refractory fiber articles in a continuous or semicontinuous process due to difficulties in controlling gelation. For example, the most efficient and desirable method for forming a refractory fiber board is to slurry fibers into a binder system and pour the slurry into a mold which permits excess liquid to drain from the fibers thus forming the refractory fiber board article. Continuous slurrying of fibers into a binder which will gel with time is not practical since it is difficult to prevent the gelation in the slurrying tanks or in impregnating or dipping tanks. Furthermore, if gel time is substantially increased, there is a corresponding undesirable increase in process time.
Furthermore, as indicated in U.S. Pat. No. 3,551,266, the use of the refractory fiber composition disclosed in the patent can be applied to produce a thickness of up to only about 1/2 inch since thicker layers of the material tend to crack.
In addition, it has been found by the Applicants herein that at colloidal silica concentrations high enough to gel upon application, resulting fiber article are too dense for obtaining the best insulation results.